Timing & Synchronisation with Atomic Clocks

Patrick Gill National Physical Laboratory

Cambridge Wireless, Location Special Interest Group 1st May 2019 Outline

• Realisation of the SI second & Cs fountain microwave clocks • Redefinition in terms of an optical clock? • Applications & clock performance requirements • Time, frequency & synchronisation dissemination routes • Microwave transfer • Optical transfer (optical fibre & free space) The Cs Fountain clock for Definition and Realization of the Second Today’sThe second best isrealization the duration cloudof 9 192 of Cs 631 atoms 770 periods Atomic laserof the cooled radiation to few corresponding K fountain into magnetothe transition-optic trap between coldthe twoatoms hyperfine are then levelslaunched ~ 1 m verticallyof the ground by laser state light atomsof the undergocaesium Ramsey 133 atom. separated field excitation in microwave cavity (CGPM 1967) fraction of excited atoms are detected by laser beams

0.9

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0.5 Operational units arbitrary 0.4 Cs fountains at NPL and several other national standards laboratories 0.3 Leading systematic0.2 frequency uncertainties: (~1 – 2) x10-16 0.1 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 10s psec per day f - 9192631770 [Hz] Steers TAI and UTC, & underpins optical frequency measurements Atomic clock evolution

Time to redefine the SI second? • Optical clocks routinely achieving 10-17 - 10-18 systematic uncertainty • Clock frequencies as quantum sensors for fundamental physics and variations in fundamental constants • Opportunities in lab, distributed clock comparisons, and in space NPL reference optical clocks

Sr 698 nm clock uncertainty: 1 x10-17 Hill et al, IOP Conf series 2016 Also manuscript in prep. Hyper-Ramsey clock lock Hobson et al, PRA (RComm) 2016

E3 octupole clock systematic uncertainty: 5 x10-17 Godun et al, Phys Rev Letters 113 (2014) Absolute freq. uncertainty (via TAI): 4 x10−16 Baynham et al, J. Mod. Optics, 65 (2018) End-cap trap design: Nisbet-Jones et al. APB (2016) Roadmap to a Redefinition: • OACs (as Secondary Representations of the Second) regularly contributing to TAI • Multiple independent measurements of an OAC relative to Cs fountain (Cs limited) • Multiple OACs (different species or in different NMIs) with validated uncertainties 2 orders better than Cs • Comparison by satellite, fibre or transportation of at least 1 OAC in 3 independent NMIs with agreement to better than eg 5x10-18. • Optical frequency ratio measurement of several other OAC standards, each meas When?: at least twice by independent NMIs, with Not before CGPM 2026 agreement better than eg 5x10-18 NPL internal network for comparing microwave and optical atomic clocks Applications of atomic clocks Realisation of the second Tests of fundamental physics & searches for fundamental constant variations

UK Timescale (TAI, UTC) Position, Navigation, Timing & Synchronisation

Inertial SatNav Astronomy navigation GNSS VLBI

Internet Mobile Synchron comms High resn . radar

Power High freq grid mgt trading Position, Navigation & Timing

Low SWaP clocks for timing and synchronisation in defence and security scenarios: • Address GNSS-denied situations • Provide local timing synchronisation & holdover in ground-based mobile activities (both military and civil) • Feed into inertial navigation in naval and aerospace applications • Distributed synchronisation & holdover for comms base stations 5G communications technology

• 4G broadband speeds to 100 Mbits s-1 • 5G anticipated speeds to 10 Billion bits s-1 • Needed to cope with yet faster access to increased levels of data transfer (eg cloud computing access, internet of things, increased density and performance of mobile phones users, real-time data for autonomous vehicles, video streaming, on-line decision making (eg medical monitoring). • Possible need for wireless frequency hopping to cope with data bottlenecks arising • Likely need for higher density of base stations with improved timing resolution and synchronisation (ie better clocks) in high-density population areas NPLTime: Precise timing and synchronization for high freq financial trading

Regulation for time-stamping & synchronisation at the s level European regulation - ESMA MiFID II RTS 25 (2017)

• NPL-traceable time to city hubs via dark fibre • Service level agreement: 1 s to UTC (NPL) at client • Capability 100 ns or better Secure resilience: • Fibre delivery • GPS link • Cs clock at hub for hold-over Size matters! - But performance trade-off Miniature Cs atomic clock Caesium vapour probed by two optical frequencies separated by a tunable frequency interval close to the ground state hyperfine structure

Modulation resonant with 9.2 GHz, → Coherent population trapping (CPT) in superposition of ground states Microwave clock signal observed via modulated laser light

CPT clock signal Time & Frequency synchronisation & transfer routes

Satellite wave transfer TWSTFT & GNSS

Free-space line-of-sight (wave or optical)

Portable clocks

Optical fibre

Adapted from Riehle, Optica (2017) Time & Frequency sync & distribution using µwaves • Network Time Protocol (NTP): 10 msec - 100 msec • MSF 60 kHz: 10 msec ToD • Precision Time Protocol (PTP): ~ 100 nsec accuracy • GNSS: ~ 50 nsec (receiver calibrated), 10-13 freq stability over ~ day • UTC(NPL) via TWSTFT: Typically UTC + 10 nsec, 5 nsec with CsF steer • GPS PPP (2 freq, carrier phase soln): 10 psec per sec, freq stability 1x10-16 per day • GPS iPPP (10 day continuity): freq stability mid 10-17 • ACES on ISS, (ground space): 300 fsec stability @ 300 s, 6 psec day-1

GNSS ACES TWSTFT Launch 2020 Frequency transfer by fibre: options

µwave-modulated Optical Carrier

Direct Optical Carrier Transfer

Optical + W Transfer via comb bandwidth 100 MHz rep rate 150 fs pulses 30 nm comb 30 nm bandwidth bandwidth Optical fibre links between National Measurement Institutes High accuracy comparison of microwave and optical clock frequencies over dark fibre (eg NPL SYRTE PTB) • Access to dark-fibre or dark-channel fibre • Fibre phase noise (optical path length changes) due to vibrations & temp •. Phase noise cancellation by comparing optical signal retroreflected back through fibre with input signal • Possible branch out to users along fibre • Dedicated dark fibre routes rented from fibre providers Fibre link optical frequency stability ~ 10-18 @ 1000 secs with fibre phase compensation Commercial system: 1 PPS timing uncertainty: 1 psec @ 1 day (µwave) 10 MHz freq uncertainty: 3x10-17 @ 1 day High-precision synchronization of remote timing sources and large area distributed arrays for VLBI astronomy and particle accelerators

• “White rabbit” rf timing sync across particle accelerators (eg CERN) → sub-nanosec stability, nanosec sync. over ~1000 distributed nodes out to 10 km via ethernet

CERN • Intention to synchronise Square Km Array antennae by femtosecond mode-locked laser pulse train over optical fibre → fsec resolution • Master oscillator pulse train delivered by fibre & combined with remote optical pulse train or microwave signals to provide sync Time & frequency distribution & synchronisation via free-space optics

Terrestrial:

Newbury (NIST), PRX 2016

T2L2 Time transfer by laser link (distant clocks) Pulsed laser ground-to-satellite-to ground retroreflections (Jason II) distant clock timing link instability ~ few psec time transfer uncertainty < 140 psec

In space: Coherent transfer between satellites demonstrated (cf EDRS) Mobile free-space femtosecond timing synchronisation of optical clocks Bergeron et al., Nature Comms 2019

• Optical time/frequency synchronisation between ”clocks” at remote sites through turbulent varying airpaths • Time-of-flight variations & fast-varying Doppler shifts • Comb-based optical TWTFT through turbulent air • 2 sites near co-located (for ease of comparison) via retroreflector on drone • Synchronisation of timescales to < 1 femtosec • Corresponding frequencies agree at 10-18 level Micro-resonator-based frequency combs Needed for optical-microwave down conversion in compact optical clocks – use ultra-high-Q micro-resonators Development programme underway at NPL

CW laser f fpump microresonator Comb generation by cascaded 4-wave mixing

Zhang, ….Del’Haye (NPL), Optica 2019

Sub-milliwatt Microresonator Combs for Optical Clocks Some current challenges: • Development of low SWaP-C portable microwave and optical atomic clocks with Thank you for listening! improved performance and robust / turn-key operation • Optical clock systems need micro-combs The NPL Time & Frequency Department • How to deliver a wider timing and synchronisation fibre distribution network at an appropriate level to clock and timing manufacturers and users • Determination of the extent to which free space optical techniques can be developed